The present invention relates generally to electronic ballast systems, and, more particularly, to a novel two-frequency AC-AC electronic ballast system for a discharge lamp.
Electronic ballast lamps (EBLs) are in widespread use. In general, an EBL is a discharge lamp, e.g., a fluorescent lamp, which is coupled to an electronic ballast circuit (system) which converts an AC line voltage into a high frequency AC output voltage for operating the lamp, and which utilizes a lamp current feedback signal to regulate the sinusoidal lamp current.
With reference now to FIG. 1, there can be seen a block diagram of a conventional electronic ballast system 20 which receives its power from a utility AC line 22, e.g., from a standard 60 Hz residential outlet. The ballast system 20 includes an EMI filter 24 which filters out high-frequency noise from the ballast circuit. The AC power from the utility line is rectified by a rectifier 26, which produces a pulsating DC output. The pulsating DC output from the rectifier 26 is smoothed out by a high-frequency power factor correction (PFC) boost converter 28, which produces a smooth DC output with highly attenuated (i.e., low percent) ripple. The PFC boost converter 28 functions to hold constant at zero the phase angle between the current and voltage waveforms of the pulsating DC output from the rectifier 26, to thereby provide a near-unity power factor (pf). In general, to meet industry requirements, a gas discharge lamp ballast should draw power from the power line with a power factor of at least 90% and harmonic distortion of less than 20%. The smooth DC output from the PFC boost converter 28 is then converted by a high-frequency DC-AC inverter 30 into a high-frequency (e.g., 25-50 kHz) AC voltage which is delivered to the lamp 32 for ignition thereof. Since the input power of the system is relatively low frequency and the output power is relatively high frequency, a bulk capacitor C.sub.e is provided in the PFC boost converter 28 for energy storage, to thereby balance the input and output power. Isolation between the AC utility line input and the lamp load is provided by the inverter 30. A control circuit A is utilized to coarsely regulate the DC output from the PFC boost converter 28, and a control circuit B is utilized to control the operating frequency of the high-frequency DC-AC inverter 30, to thereby regulate the output power applied to the lamp 32.
With reference now to FIG. 2, a typical embodiment of the conventional ballast system is depicted in partial schematic, partial block diagram form. As can be seen therein, the rectifier 26 is a full-bridge rectifier comprised of diodes D1-D4. The PFC boost converter 28 includes an inductor L1 connected in series with a forward-biased diode D, and a metal-oxide-semiconductor field-effect transistor (MOSFET) switch Q connected across the circuit. The control circuit A receives a voltage signal v and a current signal i indicative of the voltage and current values, respectively, of the pulsating DC output from the rectifier 26, at first and second inputs thereof, and receives a feedback signal from the output of the PFC boost converter 28 at a third input thereof The control circuit A functions to selectively vary the ON duty ratio and/or the switching frequency of the switch Q in order to keep the voltage and current waveforms of the pulsating DC output from the rectifier 26 in phase with one another, and thus provide appropriate power factor correction.
With continuing reference to FIG. 2, the DC-AC inverter 30 is a high-frequency half-bridge DC-AC inverter which includes a transformer T which isolates the lamp 32 from the AC line voltage. The high-frequency AC power produced by the DC-AC inverter 30 is delivered to the lamp 32 as a sinusoidal current through the L-C resonant circuit comprised of the inductor Lr and the capacitor Cr. The control circuit B receives a lamp current feedback signal and, in response thereto, controls the switching frequency of the MOSFET switches Q1 and Q2 of the DC-AC inverter 30, to thereby regulate the high-frequency AC current delivered to the lamp 32.
Since a fluorescent lamp acts as an antenna at high frequencies, the lamp current frequency is limited to about 100 kHz in order to prevent emission of excessive EMI radiation from the lamp. Typically, gas discharge lamps are operated at a frequency of 50 kHz.
The conventional ballast system described above has at least one major shortcoming. Namely, the switching frequency of the DC-AC inverter is limited by the above-stated constraint on the lamp current frequency. This limitation on the switching frequency of the DC-AC inverter requires that magnetic components (e.g, inductors and isolation transformer), and other reactive elements (e.g., capacitors) be designed for &lt;50-100 KHz frequency, thereby imposing an unduly high lower limit on the size and weight of such components, thus unduly limiting the achievable miniaturization of the ballast system.
Representative conventional AC-AC ballast systems are disclosed in U.S. Pat. No. 5,002,400, issued to Nilssen and U.S. Pat. No.4,564,897, issued to Okamoto. These systems suffer from the primary shortcoming discussed above. U.S. Pat. No. 4,661,897, issued to Pitel, discloses a DC-AC power conversion system which has a DC-AC inverter provided on the secondary side of an isolation transformer. Because this is a DC-AC power conversion system, it does not have a rectification stage. Further, it does not have a power factor correction converter.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for an AC-AC electronic ballast system for a discharge lamp which overcomes the above-described major shortcoming of conventional ballast systems. In particular, there presently exists a need in the art for an AC-AC electronic ballast system in which the switching frequencies of both the PFC converter and the DC-AC inverter are significantly higher than the lamp current frequency, without degradation of lamp operating characteristics, to thereby facilitate significant reduction in the size and weight of the reactive elements, and thus, significant miniaturization of the ballast system. The present invention fulfills this need.